Magnetic resonance tomography (MR or MRT) is an imaging method for the representation of tissue in the human or animal body. MRT is based on the principle of nuclear spin resonance according to which atomic nuclei, for example the hydrogen nuclei present in large numbers in the body, have a magnetic moment. These can therefore be excited with electromagnetic radiation of a particular frequency (resonant frequency) in an applied external magnetic field and they emit this radiation shortly afterwards.
In MRT, this electromagnetic radiation is detected as a signal. The resonant frequency of the atomic nuclei is directly proportional to the applied external magnetic field. Position encoding inside an image volume is therefore achieved by applying so-called gradient fields in addition to the basic magnetic field during the measurement; these are briefly applied magnetic fields with a maximally linear gradient in the X, Y or Z direction. The gradient fields are usually generated by particular gradient coils, which are arranged inside a superconducting magnet that generates the basic magnetic field.
Another medical imaging method is positron emission tomography (PET). PET is used in particular for the representation of physiological and biochemical processes inside the body. The patient is administered a tracer with a radionuclide, which becomes distributed in the body while emitting radioactive radiation. Positron radiators are used as tracers in PET, and these emit positrons which decay into two opposite gamma quanta in the body. These gamma quanta are measured by suitable detectors, which are arranged distributed around the body. For example, the photons are collected by a matrix of scintillation crystals in which the arrival of each photon generates a light flash. These are in turn collected and amplified by photodetectors, for example photomultiplier tubes or avalanche photodiodes. Preamplification of the signals follows after each detector.
Very recently, there has been interest in combining MRT and PET in one device. The PET detectors should in this case be arranged inside an MR magnet. Examples of this are described, for example, in the article by Markus Schwaiger et al. “MR-PET: Combining Function, Anatomy, and More” Medical Solutions/Special Molecular Imaging, Siemens AG, September 2005, the entire contents of which are hereby incorporated herein by reference.
For the integration of a PET scanner in an MR device, it is currently planned to use semiconductor light sensors as PET detectors, which are arranged together with a preamplifier inside the magnet. The preamplified signal is then fed out from the magnet for further processing. This, however, entails the problem that the electrical circuits arranged in the magnet are exposed not only to the constant magnetic field but also to the time-varying magnetic fields, for example the gradient fields described above. A voltage is therefore induced in the circuits belonging to the sensor and possibly the preamplifier, which voltage generates noise signals. In particular the input circuit for the preamplifier is particularly critical in this case, since all noise signals received here are co-amplified.